Various methods and systems are known in the art for tracking the coordinates of objects involved in medical procedures. For example, U.S. Pat. Nos. 5,391,199 and 5,443,489, whose disclosures are incorporated herein by reference, describe systems in which the coordinates of an intrabody probe are determined using one or more field transducers. Such systems are used for generating location information regarding a medical probe, such as a catheter. A sensor, such as a coil, is placed in the probe and generates signals in response to externally-applied magnetic fields. The magnetic fields are generated by magnetic field transducers, such as radiator coils, fixed to an external reference frame in known, mutually-spaced locations. The sensor signals are processed in order to determine the coordinates of the probe in the external frame of reference.
Additional methods and systems that relate to magnetic position tracking are also described, for example, in PCT Patent Publication WO 96/05768, U.S. Pat. Nos. 6,690,963, 6,239,724, 6,618,612 and 6,332,089, and U.S. Patent Application Publications 2002/0065455 A1, 2003/0120150 A1 and 2004/0068178 A1, whose disclosures are all incorporated herein by reference. These publications describe methods and systems that track the position of intrabody objects such as cardiac catheters, orthopedic implants and medical tools used in different medical procedures.
In some medical applications, data is exchanged wirelessly between the external system and the intrabody object. For example, U.S. Pat. No. 6,409,674, whose disclosure is incorporated herein by reference, describes an implantable sensor device, such as a pressure monitor, which is implanted in the heart. The device wirelessly communicates blood pressure information or other physical parameters to a remote communication device. The wireless communication techniques noted in this patent include radio-telemetry, inductive coupling, passive transponders, and conductive communication using the body as a conductor. Another position tracking system that comprises wireless communication using inductive coupling is described in U.S. Patent Application Publication 2003/0120150 A1, whose disclosure is also incorporated herein by reference. The inventors describe a system in which a wireless transponder is fixed to an object. The transponder includes at least one sensor coil, in which a signal current flows responsively to sensed electromagnetic fields. A power coil receives an RF driving field and conveys electrical energy from the driving field to power the transponder. The power coil also transmits an output signal responsive to the signal current to a signal receiver, which processes the signal to determine coordinates of the object.